Fiber reinforced plastics as a kind of composite material are anisotropic materials with reinforcing fibers and a matrix resin as the essential components and their physical properties in the fiber direction are greatly different from those in the other directions.
In general, the strength and elastic modulus in the fiber direction are very high but those in the other directions are low. As a widely adopted method for producing a fiber reinforced plastic material, layers of a filmy precursor called a prepreg formed by impregnating reinforcing fibers with an uncured thermosetting resin are laminated, formed and cured to obtain the intended product.
Hereinafter unless otherwise stated, the term "composite material" means a fiber reinforced plastic material obtained by laminating layers of a prepreg, and forming and hardening the laminate.
When a composite material is obtained from a prepreg, a fabric obtained by weaving reinforcing fibers is used in the prepreg, or layers of a prepreg made of reinforcing fibers arranged unidirectionally are laminated with the reinforcing fiber directions crossed, for rendering the physical properties almost isotropic in the obtained product.
However, it is known that even if these techniques are adopted, the impact resistance and other properties of the composite material are not fundamentally improved by enhancement of the strength of the reinforcing fibers, since they are affected by inter-layer fracture. The inter-layer region of the composite material is that region in the vicinity of the interface between laminated layers of a prepreg. This region has a low content of reinforcing fibers, and since the reinforcing fibers are differently oriented on both sides of the interface, fracture is liable to occur in the region. Especially a composite material with a thermosetting resin as the matrix resin is insufficient in impact resistance since the matrix resin is low in toughness. Furthermore, if a tensile load is applied onto a crossed laminate, inter-layer peeling often occurs at an end of the laminate, so the latitude in laminate composition is often limited. Various methods are proposed for the purpose of improving the physical properties in the directions other than the fiber direction, especially impact resistance and inter-layer toughness, and many techniques are proposed which use, at the inter-layer region, a material different from the matrix resin, for absorbing fracture energy.
U.S. Pat. No. 4,604,319 discloses location of a thermoplastic resin between fiber reinforced prepreg layers to achieve higher impact resistance. However, in this case, the tackiness and drapability as advantages of a thermosetting resin are lost disadvantageously.
In U.S. Pat. No. 5,028,478, the inventors disclosed a matrix resin containing fine particles made of a resin. In particular, it was disclosed that the localized existence of fine resin particles in the surfaces of layers of a prepreg improved the impact resistance of the composite material while maintaining the tackiness and drapability of the prepreg. However, this technique also has a problem that it is not easy to obtain fine resin particles.
Furthermore, fine particles are liable to go into the reinforcing fibers, and the ingress of fine particles into the fibers lowers the physical properties of the composite material. Any attempt to avoid this complicates the prepreg production process. Japanese Patent Laid-Open No. 90-32843 discloses a technique for improving the inter-layer toughness of the composite material by sticking woven fabrics on the surfaces of fiber reinforced prepregs. In general, it is generally easier to impregnate a resin into fibers than into particles, and this technique is more advantageous in this regard, but the advantage is erased by the necessity to weave the fibers. Furthermore, there is a limit in the production of a woven fabric, to the minimum areal weight achievable, and it is not possible to obtain an inter-layer material with a suitable areal weight.
A paper in "Composite Materials: Testing and Design (Seventh Conference), ASTM STP 893" at Page 256 states that the inter-layer toughness can be improved by placing a Kevlar or polyester mat between layers. However, the preparation of the mat requires the steps of cutting and mat formation subsequently to the preparation of fibers, and cannot efficiently utilize the advantage of using fibers. Furthermore, there is a limit in lowering the areal weight.
Japanese Patent laid-Open Nos. 90-32843, 92-292635, 92-292636, 92-292909, 92-325527, 92-325528, 92-32529 and 93-17603 disclose that the inter-layer toughness of the composite material can be improved by arranging thermoplastic resin fibers in a certain direction on the surfaces of fiber reinforced prepreg layers. However, the attempt to lower the areal weight of the thermoplastic resin fibers accompanies a disadvantage that the areal weight, i.e., performance becomes non-uniform in the width direction of the prepreg. Moreover when reinforcing fibers arranged in one direction are used, arranging the thermoplastic resin fibers in parallel to the reinforcing fibers allows the thermoplastic resin fibers go into the reinforcing fibers, and so impair the physical properties of the composite material.
These techniques have still respective disadvantages that the effect of improving impact resistance is insufficient and that impact resistance is improved at the sacrifice of other properties such as inter-laminar shear strength and handling convenience.
Furthermore, compared to the conventional prepregs, these techniques have a common problem that the production process becomes complicated and difficult.
The object of the present invention is to present prepregs which can be used to produce composite materials excellent in strength, elastic modulus, impact resistance and inter-layer toughness and can be simply and easily produced, and also to present processes for producing the prepregs.